A-level Physics (Advancing Physics)/Voltage
Charge moves through a circuit, carrying energy with it. In fact, it is this energy which enables the charge to travel, as an electric current, at all. Voltage is defined as energy per. unit charge, ie.
where V = voltage (V), E = energy (joules) and Q = charge (coulombs).
There are two electrical properties which are both measured in volts (commonly abbreviated V), and so both are known under the somewhat vague title of 'voltage'.
Electromotive Force (EMF)
Electrical power sources (such as batteries) 'push' an electric current around a circuit. To do this, they have to exert a force on the electrons. This force is known as electromotive force, or EMF. The current travels around a circuit (from the negative pole of the power source to the positive) because of the difference in EMF between either end of the source. For example, the negative end of a battery may exert 9V of EMF, whereas the positive end exerts no EMF. As a result, the current flows from negative to positive.
Potential Difference
As charge travels around a circuit, each coulomb of charge carries less energy, so the voltage (relative to the power source) decreases. The difference between the voltage at two points in a circuit is known as potential difference, and can be measured with a voltmeter.
Series Circuits
In a series circuit, the total voltage (EMF) is divided across the components, as each component causes the voltage to decrease, so each one has a potential difference. The sum of the potential differences across all the components is equal to the potential difference (but batteries have their own 'internal resistances', which complicates things slightly, as we will see).
Parallel Circuits
In a parallel circuit, the potential difference across each branch of the circuit is equal to the EMF, as the same force is pushing along each path of the circuit. The number of charge carriers (current) differs, but the force pushing them (voltage) does not.
Questions
1. A battery has an EMF of 5V. What is the total potential difference across all the components in the circuit?
2. The voltages (relative to the voltage of the battery) on either side of a resistor are -6V and -5V. What is the potential difference across the resistor?
3. At a given point in a circuit, 5C of charge carry 10 kJ of energy. What is the voltage at this point?
4. Why do the electrons move to a point 1cm further along the wire?